Scanning probe microscopy (SPM) is a branch of microscopy that forms images of surfaces of a sample or specimen using a physical probe that scans the sample. An image of the surface is obtained by mechanically moving the probe in a raster (line by line) scan of the sample and recording the interaction between the probe and the sample surface as a function of the position of the probe relative to the surface of the sample. An atomic force microscope (AFM) is a high-resolution (nanometer resolution) type of SPM. Referring now to FIG. 1, a prior art AFM is illustrated. The AFM of FIG. 1 comprises a controller 12, a microscale cantilever 14 with a sharp tip (probe) at one end that is used to scan the surface of the sample 18, a laser 20, a photo-diode 22 (while the term photo-diode is used herein, an array of photo-diodes is typically used), and an image processor 24. The cantilever tip is often, but not necessarily, constructed of silicon. When the probe tip is brought into proximity of the sample surface, forces between the tip and the sample cause a deflection of the cantilever, and this deflection may be measured using the laser and photo-diode. A laser spot (illustrated by line 28) is reflected (illustrated by line 30) by the top of the cantilever onto the photo-diode, and the resulting electrical signal (illustrated by line 32) is sent to the controller 12. The deflection of the cantilever causes an amplitude change in the reflected laser spot (and in the resulting electrical signal), such that the amplitude at any specific time corresponds to the surface contour of the sample at a specific location on the sample. As this scanning is performed over the entire surface of the sample, the resulting amplitude data corresponds to the surface contour of the entire sample. The location and amplitude data is provided to the image processor (illustrated by line 34), such that the image processor is able to create an image of the surface of the sample.
The primary modes of operation of an AFM are static mode and dynamic mode. In the static mode operation, the static tip deflection is used as a feedback signal and the force between the tip and the surface is kept constant during scanning by maintaining a constant deflection. In the dynamic mode, the cantilever is externally oscillated at or close to its resonance frequency by an oscillator, such as piezoelectric stack 16. The oscillation of the piezo stack is controlled by a drive signal (illustrated by line 26) from the controller 12, with the frequency of the drive signal corresponding to the desired oscillation. The oscillation amplitude, phase and resonance frequency are modified by tip-sample interaction forces, and these changes in oscillation with respect to the external reference oscillation provide information about the sample's characteristics. Schemes for dynamic mode operation include frequency modulation and the more common amplitude modulation. In frequency modulation, changes in the oscillation frequency provide information about tip-sample interactions. Frequency can be measured with very high sensitivity and thus the frequency modulation mode allows for the use of very stiff cantilevers. Stiff cantilevers provide stability very close to the surface and, as a result, provide true atomic resolution in ultra-high vacuum conditions. In amplitude modulation, changes in the oscillation amplitude or phase provide the feedback signal for imaging. In amplitude modulation, changes in the phase of oscillation can be used to discriminate between different types of materials on the sample surface. Amplitude modulation can be operated either in the soft contact (non-linear) or in the hard contact (linear) regime. In ambient conditions, most samples develop a liquid meniscus layer. Because of this, keeping the probe tip close enough to the sample for short-range forces to become detectable while preventing the tip from sticking to the surface presents a major hurdle for the soft contact static mode in ambient conditions. Dynamic contact mode (also called intermittent contact or tapping mode) was developed to bypass this problem. In tapping mode, the cantilever is oscillated such that it comes into contact with the sample, and the cantilever drive provides a restoring force that causes the cantilever to oscillate about a setpoint separation between the cantilever tip and the sample surface.
The rapid development of new materials produced by the embedding of nanostructural constituents into matrix materials has placed increased demands on the development of new measurement methods and techniques to assess the microstructure-physical property relationships of such materials. Although known AFM techniques are available for surface characterization, methods to assess deeper (subsurface) features at the nanoscale are lacking.